The present invention relates generally to satellite communication systems, and more particularly, to a low voltage optical phase modulator.
Satellites and other spacecraft are in widespread use for various purposes including scientific research and communications. Many of these missions, however, cannot be fulfilled without high bandwidth wireless communication. In many applications, such as business information-exchange satellite systems, the satellite relies upon wireless communication to send and to receive huge amounts of electronic data at very high data rates. Without high bandwidth wireless communication, proper satellite function is hindered and at times adversely effected. Additionally, while these communication systems are well suited for satellite use, they may also be used in other terrestrial applications.
Optical intersatellite laser communication or LASERCOM systems using a digital phase shift key data format require an optical phase modulator capable of achieving a 180 degree phase shift with as small a switching voltage as is technically feasible. LiNbO3 modulators have been developed with switching voltages Vxcfx80 that are approximately six volts in order to produce a 180 degree phase shift. Unfortunately, this voltage is not as low as would be desired.
For compatibility with space qualifiable electronics and to achieve the required bandwidth at higher data rates, switching of two volts or less is required. Because such a low voltage is difficult to achieve using LiNbO3 technology, alternative technologies have been examined. A disadvantage of most prior art technologies is that a relatively high input voltage swing is needed to produce the desired phase shifts. LiNbO3 phase shifters with a Vxcfx80 of about three volts have been demonstrated in laboratory settings. Also, prior implementations have only been concerned with producing phase shifts of millimeter wavelength carriers.
Optical phase shifters using GaAs for the electrooptical material are also commercially available. Vxcfx80 for these devices is about five volts to achieve a 180 degree phase shift. Phase shifters using electrorefraction in semiconductor quantum wells at wavelengths near the optical absorption edge have been demonstrated with a Vxcfx80 of several volts. Unfortunately, these devices suffer from having high attenuation from the large background absorption.
The disadvantages associated with these conventional optical-phase modulation techniques have made it apparent that a new technique for modulation is needed. The new technique should allow optical-phase modulation at a large bandwidth parenthetically greater than 20 GHz but require a voltage swing of less than two volts. Additionally, the new technology should allow superior reliability and performance. The present invention is directed to these ends.
It is, therefore, an object of this invention to provide an improved and reliable low voltage optical phase modulator. Another object of the invention is to allow bandwidth operation of greater than 20 GHz while requiring a voltage swing of less than two volts.
In accordance with the objects of this invention, a low voltage optical phase modulator system is provided. In one embodiment of the invention, a low voltage optical phase modulator includes a splitter having an input, a first output, and a second output. The input receives an optical signal and is split between the first and second outputs. A phase adjustment element is coupled to the second output and produces a predetermined optical shift in the optical signal to produce a phase-shifted optical signal. A first electroabsorptive element is coupled to the first output and blocks transmission of the optical signal when the first electroabsorptive element is activated with a low voltage. A second electroabsorptive element is coupled to the phase adjustment element and blocks transmission of the phase-shifted optical signal when the second electroabsorptive element is activated using the low voltage. An optical combiner having a first combiner input, a second combiner input, and a combiner output is coupled to the first and second electroabsorptive elements and receives the optical signal and the phase-shifted optical signal. The combiner combines these optical signals to produce a binary encoded optical signal.
The present invention thus achieves an improved low voltage optical phase modulator. The present invention is advantageous in that substantially larger than 180 degrees phase shift can be achieved with the same low voltage by using optical paths having longer lengths.